This displays the distribution of overlapping digrams. (The *Digram 
Distribution* button on the *Hill Cipher* page displays distribution for 
non-overlapping digrams.)

<br>It also prints the mesaure of digram roughness, which is the IC for 
digrams, the probability two digrams chosen at random will be the same.

<br>This value is about .0069 for a typical English text and about .0015 for 
a flat distribution, although the values vary quite a bit, particularly 
for small ciphertexts.
